Fast versus slow infusion of 20% albumin: a randomized controlled cross-over trial in volunteers

Background We investigated whether plasma volume (PV) expansion of 20% albumin is larger when the fluid is administered rapidly compared with a slow infusion. Methods In this open-labeled randomized interventional controlled trial, 12 volunteers (mean age, 28 years) received 3 mL/kg of 20% albumin (approximately 225 mL) over 30 min (fast) and 120 min (slow) in a cross-over fashion. Blood hemoglobin and plasma albumin were measured on 15 occasions during 6 h to estimate the PV expansion and the capillary leakage of albumin and fluid. Results The largest PV expansion was 16.1% ± 6.5% (mean ± SD) for fast infusion and 12.8% ± 4.0% for slow infusion (p = 0.52). The median area under the curve for the PV expansion was 69% larger for the fast infusion during the first 2 h (p = 0.034), but was then similar for both infusions. The half-life of the PV expansion did not differ significantly (median, 5.6 h versus 5.4 h, p = 0.345), whereas the intravascular half-life of the excess albumin was 8.0 h for fast infusion and 6.3 h for slow infusion (p = 0.028). The measured urine output was almost three times larger than the infused volume. The plasma concentration of atrial natriuretic peptide (MR-proANP) accelerated the capillary leakage of albumin and the urine flow. Conclusions The intravascular persistence of albumin was longer, but the fluid kinetics was the same, when 20% albumin was infused over 30 min compared with 120 min. We found no disadvantages of administering the albumin at the higher rate. Trial registration EU Clinical Trials Register, EudraCT2017-003687-12, registered September 22, 2017, https://www.clinicaltrialsregister.eu/ctr-search/trial/2017-003687-12/SE Supplementary Information The online version contains supplementary material available at 10.1186/s40635-022-00458-3.

Page 2 of 16 Zdolsek et al. Intensive Care Medicine Experimental (2022) 10:31 The rate of infusion may also be important for clinical efficacy. Statkevicius et al. recently compared 5% albumin administered over 30 min and 180 min after surgery and found the rapid infusion to be more effective [5]. The situation, however, remains unclear for 20% albumin, which exerts a pronounced volume effect [6,7]. As 20% has a high oncotic pressure, a fast infusion might cause a more pronounced rise in plasma oncotic pressure resulting in intravascular fluid overload due to a powerful recruitment of extravascular fluid [6]. However, hormonal adjustment might counteract such overload. For example, a fast infusion might stimulate release of atrial natriuretic peptide (ANP) from the heart, which increases the capillary permeability and accelerates the urine flow [8].
The objective of the present study was to compare the plasma dilution, which mirrors the plasma volume expansion, when 20% albumin is infused at different rates in volunteers. Another objective was to investigate how much the recruitment of extravascular fluid prolongs the plasma dilution for the two infusion rates.
Our primary hypothesis was that a fast infusion increases the plasma volume more than a slow infusion. The rationale is that that elimination of 20% albumin is slow [7] and, therefore, one could anticipate a greater intravascular volume expansion when given fast. A secondary hypothesis was that recruitment of fluid from the extravascular space would markedly prolong the half-life of the plasma volume expansion. This could occur if extravascular fluid was continuously recruited rather than only during the infusion.
The study methods consisted of mass balance calculations and volume kinetic analysis of the albumin and fluid components of 20% albumin. We measured the midregional pro-atrial natriuretic peptide (MR-proANP) to control for the role of ANP in this setting. The results should have clinical applicability, as the kinetics of 20% albumin is quite similar in volunteers as in clinical patients with and without traumainduced inflammation [7,9].

Materials and methods
This study is an open-labeled interventional randomized unblinded controlled trial with cross-over design where 12 healthy participants (six male and six female) underwent two infusion experiments, 3-20 weeks apart, where they received 3 mL/kg of 20% albumin by intravenous infusion during either 30 min ("fast infusion") or 120 min ("slow infusion").
The Regional Ethics Committee of Linköping (Dnr 2017/478-31) approved the study. The Swedish Medical Products Agency (Eudra-CT 2017-003687-12) also approved the protocol. The project also included a study arm with iso-oncotic albumin (clinicaltrials.gov NCT03453320), but the control groups differed due to logistical issues and the results will be presented elsewhere.
All participants gave informed consent orally and in writing. Inclusion criteria were an age between 18 and 60 years, and absence of medical disease and medication. Exclusion criteria were pregnancy, difficulties with placement of venous cannulas, and severe allergy. The study was conducted in compliance with the Declaration of Helsinki. Procedure A statistician prepared 12 envelopes for randomization of the participants to start with either the slow or the fast infusion. The envelopes were opened one day prior to the first infusion by the investigators. All participants fasted from midnight before the study and throughout the study period. However, they were allowed to drink 2 dL of liquid and eat one sandwich 1.5 h (h) prior to arrival at the research faculty at the hospital.
A venous cannula was placed in the right arm and another in the left for blood sampling and fluid infusion, respectively. The participants emptied their bladders 30 min before the study started and were then placed in a supine position until the end of the experiment. Baseline samples were withdrawn, and the participants then received the infusion. Blood was sampled on 15 occasions over a period of 6 h.
When albumin was administered over 30 min, the excreted urine was measured and sampled just before the infusion was initiated, at 30 min after the infusion ended, and at 6 h. When albumin was administered over 2 h, the excreted urine was measured and sampled just before and at the end of the infusion, and at 6 h.

Blood and urine analyses
Whole blood was analyzed for hemoglobin (Hgb) concentration and hematocrit with a coefficient of variation (CV) of 1.0%, as given by duplicate samples at baseline, using a Cell-Dyn Sapphire instrument (Abbott Diagnostics, Abbott Park, IL, USA). Plasma was analyzed for albumin, creatinine, sodium, and potassium on a Cobas 8000 system (Roche Diagnostics, Basel, Switzerland) at the hospital's certified central laboratory (CVs were 2.3%, 1.9%, 0.7%, and 1%, respectively, as given by the laboratory).
The plasma colloid osmotic pressure (COP) was measured in our research laboratory on an Osmomat 050 device (Gonotec, Berlin, Germany) with a CV of 2%.
The plasma concentration of the mid-regional pro-atrial natriuretic peptide (MR-proANP) at baseline and 30 min after the infusion ended was analyzed by radioimmunoassay (Brahms MR-proANP Kryptor, Henningsdorf, Germany) with a CV of < 3.5%. The manufacturer reports a median value in healthy humans of 46 pmol/L.
Urine was analyzed for creatinine on the Cobas 8000 system with a CV of 1.9%.

Mass balance
The baseline plasma volume (PV o ) was estimated from the height and weight of the participants, as suggested by Nadler et al. [10]. The PV change to a any later Time t of the experiment (PV t ) was calculated based on the hemodilution curve, with correction for sampled blood volume, as described previously [6].
The albumin mass was taken as the product of the plasma volume (PV) and the plasma albumin concentration (P-Alb). Multiplication with PV is necessary because P-Alb is diluted by the infused fluid volume and by the oncotic-driven recruitment of extravascular fluid, which gives an unbalanced relationship between P-Alb and the intravascular albumin mass. Capillary leakage. The net capillary leakage of albumin was obtained as the change in albumin mass, with correction for the infused amount of albumin, between baseline (time 0) and a later time t [7]. The following equation was used: Half-life. The half-life of the infused albumin was obtained from the logarithm of the slope of the albumin mass, given as [(1 + PV dil )P-Alb t −P-Alb o ] versus time, when an apparent first-order elimination had been established post-infusion [7]. For each experiment, the half-life of the decay of the PV expansion was estimated in the same way for the albumin mass.

Albumin and fluid kinetics
A one-compartment model was used to study the kinetics of the infused albumin mass throughout the entire experiment. In this model, albumin mixed in fluid was infused at a rate R o into a central body fluid space V c , which was then expanded to v c . The net capillary leakage was given by the rate constant k b ("net" denotes that k b represents the true capillary leakage minus the albumin added via the lymph). The dependent variable was the product of the increase in measured P-Alb and the plasma dilution; the latter was given by ((Hgb o /Hgb t ) − 1)/(1 − hematocrit o ), where the subscript o denotes the baseline and t a later time. Minor correction of the plasma dilution for blood sampling was made [6].
The kinetics of the infused fluid volume was evaluated using a model with microconstants that is developed for studies of 20% albumin [11]. This model has one infusion, one absorption route, and two elimination routes and was fitted to the plasma dilution and the urine output, which served as the dependent variables.
Absorption occurred from an extravascular source, which is likely to be the interstitial fluid space, by (supposedly) oncotic forces and at a rate that was determined by a constant denoted k 21 . The interstitial fluid volume at baseline (ICF o ) was assumed to contain fluid accounting for 15% of the body weight [12].
Fluid volume was eliminated by urine output (k 10 ) and capillary leakage (k b ). This "base model" was expressed by the following differential equations: where V c is the baseline and v c the expanded central volumes, and U the measured urine output. The rate parameter k 21 does not come into play before the infusion begins. The fixed parameters in the albumin model (V c and k b for albumin) were estimated simultaneously for all 24 experiments using the first-order conditional estimation extended least-squares (FOCE ELS) search routine in the Phoenix software for nonlinear mixed effects (NLME), version 8.2 (Pharsight, St. Louis, MO) and the additive model for the within-subject variability. The dependent variable was P-Alb corrected for plasma dilution. The fixed parameters in the fluid model (V c , k 10 , k b , and k 21 ) were estimated in the same way. Here, the dependent variables were the frequently measured plasma dilution and the urine output measured at 1 h and 6 h.
Both base models were both refined by adding individual-specific covariates. Nine potential covariates were examined. Age, body weight, gender, Hgb o , and urine osmolality and urine creatinine at baseline were entered once for each patient. Plasma creatinine, and MR-proANP were measured twice per experiment and applied at the point in time when measured. The change in plasma albumin from baseline was entered as a time-varying covariate 15 times per experiment (at the same time points as Hgb was measured).

Outcome measures
The primary outcome measure was the plasma dilution, which equals the relative change in plasma volume, at 6 h and at the end of the infusions of hyper-oncotic albumin. Secondary outcome measure was the intravascular half-life of the excess amount of albumin and the plasma dilution as obtained by mass balance and kinetic analysis.

Statistics
Power analysis prior to the study was based on the previously obtained mean ± standard deviation (SD) values of 15.8% ± 4.9% for the plasma volume expansion at end of infusing 3 mL/kg of 20% albumin [11]. We aimed at identifying a difference in plasma volume expansion of 20% at the p < 0.05 level and with a certainty of 80%. This calculation yielded 22 experiments.
The measured variables were reported as the mean ± SD and, when appropriate, as the median and interquartile range (IQR). Differences between the two infusions were studied, depending on the distribution of the data, by the paired t test or Wilcoxon's matched-pair test. p < 0.05 was deemed statistically significant.
The excess amount of intravascular albumin and the plasma volume expansion over time was expressed as the area under the curve (AUC) which was calculated by the linear trapezoid method.
Kinetic parameters were reported as the best estimate and 95% confidence interval (CI). A new parameter (fixed or covariate) was accepted if its 95% CI did not include 0 and the inclusion decreased the − 2 log likelihood (− 2 LL) for the model by > 3.8 points (p < 0.05).

Results
The participants were studied between February 2018 and November 2018. One participant was excluded due to difficulties with the placement of the two venous cannulas before the infusion was administered. This subject was replaced by another volunteer. The finally included volunteers were 28 ± 10 years old, had a body weight of 75 ± 10 kg, and body mass index of 24.2 ± 2.8 kg/m 2 . A CONSORT flow diagram is shown in Fig. 1.
The measured blood Hgb and plasma albumin concentrations are shown in Fig. 2 and the results of other measurements and calculations in Table 1. The mean (SD) MR-proANP was 43 ± 12 and 44 ± 17 pmol/L, respectively, before the fast and slow infusions were started. Thirty minutes after they ended, these mean (SD) concentrations had increased to 54 ± 24 and 55 ± 24 pmol/L, respectively. The change was significant by p < 0.01.

Mass balance
The largest mean (SD) plasma volume expansion during the fast infusion experiment was 16.1% ± 6.5% and occurred 10 min after the infusion ended. The corresponding maximum value for the slow infusion was 12.8% ± 4.0 (p = 0.52) and recorded at the end of the infusion (Fig. 3A). The plasma volume expansion at 6 h was 4.5 (IQR 2.0-8.4)% after the fast infusion and 5.2 (1.4-12.6)% after the slow infusion (p = 1.0). The plasma albumin concentration was 40.0 ± 2.2 g/L (mean ± SD) at baseline prior to the fast infusion and 40.0 ± 2.0 g/L prior to the slow infusion. These concentrations had increased by 6.6 ± 2.3 g/L and 7.1 ± 2.3 g/L, respectively, at the end of the infusions (p = 0.62; Fig. 3B).
The plasma COP was 25.0 ± 1.2 prior to the fast infusion and 25.1 ± 1.4 prior to the slow infusion. The increase in COP was 2.7 ± 1.5 mmHg at the end of the fast infusion and 3.0 ± 1.1 mmHg after the slow infusion (p = 0.65; Fig. 3C).
At the end of infusion, the fast infusion had increased the plasma volume by twice the administered fluid volume, while the slow infusion had expanded the plasma volume by 1.6 times the infused volume (p = 0.19; Fig. 3D).
At 6 h, the net capillary leakage of albumin amounted to 41% ± 16% of the infused amount after the fast infusion and 46% ± 14% after the slow infusion (p = 0.53; Fig. 3E).
The median AUC for the excess amount of albumin during the first 2 h of the experiments was 78% larger for the fast infusion (p < 0.003) and the plasma volume expansion was 69% larger for the fast infusion as compared to the slow infusion (p < 0.034; Table 1). After the first 2 h the plasma volume curves were virtually identical, while plasma albumin was slightly more increased after the slow infusion (Fig. 3A, B). The AUC for the plasma volume expansion during the entire experiment still did not differ significantly ( Table 1).
The cumulative urinary output at 6 h amounted to 631 ± 354 mL and 612 ± 242 after the fast and slow infusion experiments, respectively (p = 0.83), which represented 2.9 ± 1.6 and 2.8 ± 1.2 times the infused volume.

Kinetic analyses
Relevant graphic output of the analysis of the kinetics of the infused albumin is shown in Fig. 4 (for underlying data, Table 2, top). Covariance analysis showed that the net capillary leakage of albumin, as represented by the rate constant k b , was accelerated by plasma MR-proANP in the high range (Fig. 4C) and by low Hgb at baseline (Fig. 4F).
Graphic output from the analysis of fluid kinetics is given in Fig. 5 (for underlying data, Table 2 The covariance analysis confirmed that the intensity of the fluid recruitment by k 21 was directly dependent on the elevation of plasma albumin from baseline that resulted from the infusion of 20% albumin. The covariance analysis also showed that the urine flow was accelerated by a high plasma MR-proANP concentration (Fig. 5C) and a low urinary creatinine concentration at baseline (Fig. 5F).
The influence of the covariates on the plasma volume expansion is illustrated by simulations in Fig. 6. Factors of importance to the plasma volume expansion. Computer simulations contrasting A rates of infusion B excessive (× 2) or reduced (× 0.5) increase in plasma albumin relative to the measured concentrations C the plasma MR-proANP concentration was high or low D whether the baseline urinary concentration of creatinine was high or low. The data from Table 2 were used. Simulations were performed by setting all kinetic parameters to the mean value except for the parameter that was varied. Subplots B-D were based on the kinetic data from all 24 experiments, but the plasma albumin measured during the 30-min infusion only

Key results
The results show that 20% albumin was an effective plasma volume expander regardless of infusion rate. The fast infusion expanded the plasma volume much more at 30 min, but only 25% of the slow infusion had been administered at that time. The area under the curve for the plasma volume expansion during the first 2 h was still 65% larger for the fast infusion and was not compensated by a less pronounced long-term expansion (primary hypothesis), which would else be expected according to pharmacokinetic theory. Hence, at 6 h the plasma volume was virtually identical for the two infusion rates. As much as one third of the maximum volume expansion remained in the intravascular space at 6 h.
The half-life of the intravascular persistence of the infused fluid volume was prolonged from 0.5 h to 5 h by gradual recruitment of extravascular fluid due to the rise in plasma albumin, which is apparently an important characteristic of the clinical efficacy of 20% albumin (secondary hypothesis).
Capillary leakage of albumin occurred continuously throughout the study, but more slowly when the infusion was given fast. The fluid volume kinetics did not differ between the two infusion rates, but it appeared to be more strongly governed by individual-specific physiological factors (covariates) than the capillary leakage of albumin.

Diuresis and MR-proANP
The marked reduction of urinary creatinine during the experiment (Table 1) illustrates that 20% albumin is a diuretic. The volume of excreted urine was almost three times larger than the infused fluid volume.
The urine output was affected by two individual-specific factors. The first factor was the urinary creatinine concentration, which is high when the kidneys concentrate the urine to retain water (Fig. 6D). We have observed the same retarding effect of urinary creatinine on the excretion of infused crystalloid fluid [13]. High urinary creatinine is caused either by acute dehydration or a low habitual intake of water [14]. The latter possibility should be at hand in the present study as the subjects were healthy volunteers.
The second factor was the plasma MR-proANP concentration (Fig. 6C). Prior to this study, we had concerns that the rapid infusion of 20% albumin would increase the plasma volume sufficiently to markedly elevate the MR-proANP concentration, which is a precursor of ANP that is excreted from the atrium of the heart in response to distention (increased cardiac preload). The increase reached only 25% in both groups, but the between-patient variation in MR-proANP was still large enough to render this variable a statistically significant predictor of the urine flow.
Statkevicius et al. recently compared slow and rapid infusions of 5% albumin after abdominal surgery and reported a greater increase in MR-proANP for a 30 min infusion than for a 180 min infusion. However, they found no difference in the transcapillary escape rate of albumin [5].
The plasma concentration of ANP doubles in response to rapid infusion of a crystalloid fluid [15,16]. The urine flow increases also in response to modest elevations of brain natriuretic peptide, which is closely related to ANP [17]. This hormone is assessed